In a brazing apparatus for aluminum products and etc., an atmosphere with a low oxygen concentration is necessary when heating an object to be processed. For realizing such an atmosphere with low oxygen concentration, it has been proposed to keep low the oxygen concentration in a nitrogen atmosphere in a continuous furnace for brazing aluminum products, by contacting an furnace inner wall of a preheating furnace made of a carbonaceous material with oxygen, and reacting the oxygen therewith to change it into CO, wherein when preheating the aluminium products up to about 500° C. in the preheating furnace, heating efficiencies were improved by circulating the nitrogen atmosphere in the preheating furnace by convection using a fan, and the oxygen was introduced into the preheating furnace, and was diffused in the nitrogen atmosphere by the fan (see Japanese unexamined Patent Publication No. 2004-050223).
It has also been proposed to improve qualities thereof by lowering the oxygen concentration using a carbon muffle, i.e., employing a heating zone of the continuous furnace for brazing which has the carbon muffle in the furnace wall, and has a heating chamber in which a furnace atmosphere gas is provided. In other words, it has been proposed to obtain the function of decreasing oxidation of a flux or aluminum products, by arranging as the carbon muffle a graphite outer muffle (i.e., a tunnel-shaped muffle formed with carbon material) having a rectangle cross-section, and forming CO+CO2 in the furnace by the graphite outer muffle (see Japanese unexamined Patent Publication No. 2008-105044).
In addition, as a method of lowering the oxygen concentration in the furnace, it has been proposed to increase the furnace pressure, by filling a non-oxidizing or reducing atmosphere gas in a furnace composed of a plurality of zones, and atmospherically cutting off each zone and the inside and outside of the furnace (see Japanese unexamined Patent Publication No. 2-238289).
When the temperature in the furnace is higher than a predetermined temperature, a reducing reaction of oxygen rapidly proceeds, however, when the temperature in the furnace is lower than the predetermined temperature, the reducing reaction of oxygen does not easily proceed. In addition, when the inside of the furnace is not heated, the reducing reaction, as a matter of course, almost does not proceed. Therefore, when starting-up the furnace, even when there is no object to be processed in the furnace, it is necessary, while heating, to flow a gas such as N2 or Ar into the furnace, and expel the oxygen from the furnace, until the oxygen concentration of the atmosphere in the furnace is lowered to a desired concentration. For example, a brazing furnace takes half a day or more to start-up, i.e., it takes a long time and energy.
Further, in a method of increasing the inner pressure of a furnace divided into a plurality of zones by filling the furnace with a non-oxidizing or reducing atmosphere gas, the productivity is poor, and the cost is high, since the object to be processed is intermittently fed to the furnace.
In the case of a batch furnace, in order to lower the oxygen concentration of the atmosphere in the furnace, it is necessary to depressurize the inside in the furnace to vacuum once, and then substitute the atmosphere thereof with a gas such as N2 or Ar. Therefore, the inside of the furnace needs to be depressurized to vacuum for each batch, and thus the workability of this case is also poor. In addition, the oxygen concentration after lowering the pressure depends on the equipment used, or the oxygen concentration in the gas used, and thus, if a very low oxygen concentration is needed, a highly pure gas, such as Ar or N2, must be used. As a result, the processing cost thereof increases.